As a semiconductor device used in power electronics, a vertical device is primarily used. The vertical device includes a main electrode on each of two opposing main surfaces of a semiconductor substrate, and a main current flows in a direction perpendicular to the main surfaces of the semiconductor substrate. In a usual vertical metal oxide semiconductor field effect transistor (MOSFET), when the MOSFET is turned off; a depletion layer extends in a drift layer, and the depletion layer serves as a voltage blocking layer. When the device is turned on, a current flows through the drift layer provided on the semiconductor substrate. The drift layer, which has higher resistance than that of the semiconductor substrate, is one of major resistance components of the device. Therefore, a smaller thickness of the drift layer can reduce drift resistance, and can reduce substantial on-resistance of the MOSFET. Further, on-resistance can also be reduced by increasing impurity concentration in the drift layer.
The breakdown voltage of the semiconductor device is determined by the width of, the depletion layer in the drift layer. That is, a smaller, thickness of the drift layer reduces a breakdown voltage of the semiconductor device. Further, higher impurity concentration of the drift layer also reduces the width of the depletion layer, and thus reduces a breakdown voltage of the semiconductor device. In this manner, a trade-off relationship exists between a breakdown voltage and on-resistance.
As a structure for improving the trade-off relationship between a breakdown voltage and on-resistance, a super junction structure is proposed. The super junction structure is a structure in which, in the drift layer, p-type impurity layers (hereinafter referred to as p-type pillar layers) and n-type impurity layers (hereinafter referred to as n-type pillar layers) are alternately arrayed in a direction orthogonal to the direction in which the main current flows.
In such an SJ structure, when the drift layer has a conductivity type of an n type, a depletion layer extends from a pn junction surface between the p-type pillar layer and the n-type pillar layer, as well as from a pn junction surface or a metal junction surface present on a surface of the semiconductor device. That is, a depletion layer having the same depth as the depth of the pillar layer is formed in the drift layer. With this, even when impurity concentration of the n-type pillar layer, i.e., impurity concentration of the drift layer, is high concentration, the concentration is equal to impurity concentration of the p-type pillar layer, and the n-type pillar layer is thereby fully depleted. Thus, the breakdown voltage can be maintained. As a result, the trade-off relationship between a breakdown voltage and on-resistance in the semiconductor device is drastically improved, and drift resistance can be lowered.
A semiconductor device having the SJ structure has an important problem as to how to secure a breakdown voltage in a termination region that is a peripheral portion of an element region (active region). A low breakdown voltage at a termination part reduces a breakdown voltage of the entire semiconductor device. Further, concentration of an avalanche current at the termination part may cause malfunction of the semiconductor device.
In general, impurity concentration of the p-type pillar layer and the n-type pillar layer in the termination region is set to be lower than impurity concentration of the p-type pillar layer and the n-type pillar layer in the element region. This is because when the impurity concentration in the termination region, in particular, donor concentration in the n-type pillar layer, is reduced to be low, a breakdown voltage in the termination region is improved.
Patent Document 1 discloses a technology of a multi-epitaxial method in a silicon semiconductor device where epitaxial growth and ion implantation are repeated to form a large number of epitaxial growth layers in a drift layer, thus forming the SJ structure. Here, impurity concentration in a termination region is set to be lower than impurity concentration of an active region. Such a configuration, however, cannot be produced with an SJ structure that is formed with a highly productive trench filling method, and is not suited for a semiconductor such as silicon carbide that has a small diffusion coefficient of impurity.